Rent or borrow an overhead projector.
Scan your image into a computer scanner and save the image to print on a computer printer or print directly from the scan if you have a printer/scanner combination. Print the image onto the transparency sheet. Printable transparency sheets are available at office supply stores.
Plug in the overhead projector and place your image on the glass panel of the overhead projector. Turn out the lights and project the image onto the wall where desired.
Adjust the overhead projector to create the size mural you want. Move the projector closer to the wall for a smaller image/mural and move the projector farther away from the wall to create a larger image/mural. Follow the overhead projector manufacturer's recommendations for the best use of your particular make and model.
Choose a section of the mural to begin your work. Begin tracing the projected image with your pencil directly onto the wall. Line drawings such as images from coloring books work the best due to their bold, black outlines, but any image is traceable with patience.
Trace the entire image until completed and then turn off the projector. Turn on the lights. Move the projector out of your way so you will not trip on the cord, or get paint, etc., onto the overhead projector.
Paint the mural with acrylic or latex-based paints and the appropriate-sized paintbrushes for the area of wall being covered. Small brushes work best for detailed work. Paint according to the colors of the original image or according to your own color scheme. Erase pencil marks as you paint, or simply paint over them.